1. Field of the Invention
The present invention is directed to a method for transmitting data trains, and an apparatus for performing the method of transmitting data trains over a communications link.
2. Discussion of Background and Relevant Information
As illustrated in FIG. 4, when data terminal equipment 200, such as, for example, a computer, is used to transmit digital data to another computer over a communications link, such as, for example, a telephone network 210 or a radio relay 220, the digital data is usually converted to a low-frequency analog signal. While digital communication networks exist, the digital data usually is converted from a digital data stream to an analog data stream because the vast majority of communication networks operate in the analog domain. The device that is used to execute this conversion process is referred to as a modulator-demodulator (also known as a modem 230). The modem 230 serves to convert serial binary digital data to and from a signal form that is appropriate for a respective communication channel, such as the telephone network or radio relay.
The analog frequency of the converted digital data is changed, or shift-keyed, between two values fh and ft (where, fh&gt;ft), depending on the logical states of the inputted signal. This transmission modulation method is referred to as a frequency shift keying (FSK) modulation method.
To transmit digital data over relatively long electrical, or optical, lines or by radio, the FSK-modulated low frequency signal is superimposed upon a high-frequency carrier channel. The receiving side then demodulates the high-frequency signal. The FSK-modulated low frequency signal is recovered, and then demodulated by a discriminator. Finally, the digital data stream is regenerated.
As noted above, the modulation and demodulation of the low frequency signal is typically performed by a modem. Such a modem is disclosed, for example, by P. Bocker, Datenubertragung [Data Transmission], Munich, 1976, Vol. 2, pp. 30-32. A review of page 31 and FIG. 7.10 of this document illustrates a frequency-shift-keyed oscillator that is provided with a resonant circuit, in which the natural frequency of the oscillator is varied as a function of the data to be transmitted. A review of page 32 and FIG. 7.12 illustrates a demodulator with a resonant circuit demodulator that functions to regenerate the transmitted data signal. Page 31 and FIG. 7.11 illustrates the data appearing in the modem.
According to the Bocker document, transmitted data representing a logical 1 has a frequency modulated signal of approximately 3.5 oscillations at a frequency fh, while transmitted data representing a logical 0 has approximately 1.5 oscillations at a frequency ft. Thus, the transmission rate (also referred to as the baud rate) of the transmitted data signal is approximately 3.5 times lower than frequency fh.
A high frequency signal fh leads to a correspondingly high bandwidth requirement in the modulation of the carrier channel. For a fixed bandwidth, the transmission rate is therefore limited to a relatively low value. Reducing the oscillations per data bit makes it possible to increase the transmission rate. However, if the frequency fh remains twice as high as the frequency ft, twice the bandwidth is needed to transmit the 1 data bits, as compared with that for the 0 data bits. According to the method by Bocker, the resultant transmission rate is always relatively low in comparison with the required bandwidth. Over the frequency range of a transmission system, the attenuation is often not constant and the phase often does not rise linearly with the frequency, so that signal deformations occur during transmission. If two frequencies fh and ft are used, group delay distortion occurs, leading to signal distortions that normally result in an increased error rate. Thus, the method of Bocker requires that the signal distortions be reduced using complicated and expensive equalizers.
German published, non-examined Patent Application DE-OS 2 008 897 discloses a method for transmitting binary-encoded data over a high-frequency channel, in which the binary encoding is done by a data-dependent shifting of the carrier frequency. In this document, a 0 data bit is represented by setting the carrier to a frequency f1, where f1 equals f0 and fa, while a 1 data bit is represented by setting the carrier frequency to a frequency f2, where f2 is equal to f0 minus fa, where frequencies f1, f0 and fa are distinct frequencies. This method permits one to achieve a substantially higher transmission rate in the baseband range as compared with the frequency shift keying method (described above). Radio equipment provided with carrier frequency shifting are able to transmit data signals in digital form. However, in order to accomplish this carrier shift keying, the radio must be provided with special provisions that are not normally found in commercially available equipment. Moreover, this type of modulation can have an unacceptably high error rate, because of the non-linearities and shift of a discriminator characteristic curve. For example, FIG. 1A of the present specification illustrates an upper binary signal that comprises a non-return to zero (NRZ) signal. When such a transmission contains a long series of 0 data bits or 1 data bits, the resultant signal does not provide information which is required to maintain a correct decision threshold in a discriminator. Consequently, the decision threshold may shift and 1 data bits may be detected as 0 data bits. The present invention overcomes this limitation, by employing, for example, a ternary signal, in which zero-crossings occur between two 0 data bits. Accordingly, a clock regeneration and decision threshold correction can be easily achieved.
Moreover, the method described by German published, nonexamined Patent Application DE-OS 2 008 897 requires that the binary data signal must be delivered to the modulator via a dc-coupled input. However, telephone network interface regulations of numerous countries prohibit the introduction of a dc voltage signal upon the country's communication network, so as to keep allowed frequency channels free of harmonics that would be delivered via such inputs and be modulated on the carrier channel.